The present invention relates to a vertical roller mill, and more particularly to a mill for final milling of clinker or blast furnace slag used as cement material.
FIG. 1 of the accompanying drawings shows a conventional vertical roller mill including a table 1 and a plurality of rollers 2, wherein each roller 2, which is in the general shape of a tire, has a central radial plane P that is perpendicular to the top surface of the rotary table 1 and that is perpendicular to the axis of rotation of the tire. Material to be milled or crushed is supplied to a central portion of the table 1, moved centrifugally outwardly to the rollers 2, and milled between the rollers and table.
Each roller 2 has a radial plane P located axially centrally thereof. The rollers are located in an annular groove formed in the upper surface of the table, and the clearance CL between the outer periphery of each roller 2 and the bottom of the groove is narrowest at the central plane P, where, also, the roller pressure is greatest. At substantially in the central plane P, the roller 2 and the table 1 rotate at the same circumferential speed.
This causes slips due to differences in the circumferential speed between adjacent portions of the roller and table, as shown graphically in FIG. 2, on both sides of the central plane P. The direction of the slip on one side of the plane P is opposite to that on the other side, and the slippage on the outer side of the plane P is far greater than that on the inner side. This increases the slipping force due to the roller pressure on the outer side, thereby causing an imbalance of forces on opposite sides of the plane P.
The force imbalance causes a large bending moment around the supporting pivot 5 for each roller, and requires the arm 3 which supports each roller to be very rigid. The imbalance due to the slippage also causes vibrations of the roller in the circumferential direction of the table.
This tendency is particularly strong in a roller mill for fine milling, because the clearance between the table and the rollers is narrowed to achieve an effective slipping force.
Another prior art vertical rolling mill is disclosed in Japanese Patent Provisional Publication No. 58-109146. This mill includes a rotatable table and rollers which have both circular and conical surfaces. The mill forms a region for coarse milling wherein the relative slip between the table and the rollers is reduced for less wear, and another region for fine milling wherein the relative slip is increased for higher milling efficiency.
It is a general object of this invention to provide an improved vertical roller mill, which has a region of slip suitable for producing fine milling between the rollers and the table, without causing circumferential vibrations of the rollers.